Someone posted the Mass. Casle Doctrine in the comments:
Looks cut an dry to me. He could have killed the intruder if the intruder made it inside.
I think that he would have gone to prison anyway just because it's in Mass.
Could be a situation where you could have killed the intruder but would later regret it, instead of ordering him on the ground after firing a warning shot.
I am just playing devils advocate here...
The guy who killed his neighbors intruders, in Texas, said he regretted it.
I think any human would have some degree of PTSD after that.
I wonder if there is any case law on a similar circumstance.